Yes, I guess it started with Nibali in 2018 and Valverde following suit the year after. But the changes in 2012 certainly helped make it more viable for such riders. As for the other cobbled races, E3 has always been the most Ardennes-like of the bunch.
I still don't quite agree that the distinction between classics riders and GC riders has changed much in recent years. I think Nibali and Valverde were important as trailblazers who showed that it was possible for lighter riders to do well. It did of course help that they both were excellent at bike handling and positioning compared to the average GC rider. But there's also a difference between doing well and actually winning. Pogacar is rather exceptional. I don't think many other GC riders could have won RVV like he did, even if they tried. You need a lot of explosivity which is not normally exhibited by your typical GC rider, and it's quite telling how the explosive Valverde did significantly better than the more diesel-like Nibali. I don't think we will have a rush of GC riders at the start in Antwerp anytime soon. With some notable exceptions like Pogacar, the climbers are still at such a disadvantage that it's not worth making it a goal.
Paris-Roubaix is another notch up on the difficulty scale for the GC riders compared to RVV, E3 etc. While Pogacar may well be capable of going top 10 in Roubaix as well, the physical demands are very different from RVV. You can benefit from high w/kg on the hills of RVV with the current route, but PR is much more about absolute power. And having a bit of weight helps on the Roubaix cobbles. There's a reason why you even see differences between classics specialists in how well they're suited to each of the two monuments. Degenkolb is maybe the prime example.